A conventional current mode voltage regulator includes a current feedback loop and a voltage feedback loop for controlling the peak instantaneous current through a transistor switch for each switching cycle to regulate the output voltage. The duty cycle of the transistor switch is controlled by the feedback paths to regulate the voltage. It is well known that operating at a duty cycle near or above 50% can result in oscillations of the duty cycle and output voltage in response to perturbations in the load or the input voltage. For example, in response to a perturbation, the duty cycle may continue to oscillate between two values in alternate switching cycles, referred to as a sub-harmonic oscillation.
FIG. 1 shows an example of a current mode voltage regulator without any slope compensation. The solid waveform 6 represents the instantaneous inductor current at a duty cycle of about 70% when the regulator is operating properly. The transistor switch is turned on at the beginning of a switching cycle and turned off when the instantaneous inductor current crosses a current threshold Ith, set by an error amplifier receiving an output voltage feedback signal. If there is a perturbation in the load or input voltage, the duty cycle reacts to the perturbation, which creates the dashed line instantaneous inductor current waveform 8. As seen, the duty cycle then fluctuates and may settle into an oscillation pattern where the duty cycle changes each switching cycle while trying to maintain the proper peak current. This creates significant ripple in the output voltage.
The typical current mode regulator includes a slope compensation circuit that dampens such duty cycle oscillations. In one example, the current threshold (in the voltage feedback path), used for determining when to turn the transistor switch off, is modulated by the same downward ramp signal for each switching cycle to provide slope compensation. Such a technique works but the slope compensation is fixed by the circuit designer, where the designer optimizes the slope compensation for a particular set of likely off-chip components to be selected by the user and for a particular duty cycle. Therefore, even though the duty cycle perturbations may be eventually dampened, the number of switching cycles needed for the damping in actual operation is variable. Additionally, with fixed slope compensation, the load current perturbation may be such that the duty cycle perturbation is never damped or the duty cycle goes into an oscillating pattern. This is a significant problem when a constant output voltage is important. If the slope compensation is higher than necessary, the behavior of the converter may exhibit characteristics of voltage mode control, hence voiding the advantages of current mode control.
In a similar type of slope compensation circuit, the instantaneous current feedback signal, rather than the current threshold, is adjusted by a fixed upward slope compensation. Since the transistor switch shuts off when the rising instantaneous current signal crosses the current threshold, the slope compensation can either modulate the current threshold with a downward ramp or modulate the instantaneous current signal with an upward ramp.
Other types of slope compensation circuits may be adaptive and receive information from the regulator to adjust the slope compensation to be more optimal. However, such circuits tend to be complex and hence use up valuable chip area and power. In some examples, the adaptive slope compensation circuit requires a microprocessor.
What is needed is a compact adaptive slope compensation circuit that uses very little area and power yet adjusts the slope compensation so that perturbations in the duty cycle are damped within only one cycle or only a few cycles.